Went to a camping in the Laurentians in Québec (Ste-Agathe) and there were those things near some camp sites. The sites are only accessible by footpaths. No car access. And if you bring a bike, you have to walk it up a hill, so I doubt they are bike racks.
EDIT: I have sent an email to the park and they have been kind enough to reply. It's just a type of fireplace with a grate. They didn't give more info than just this. Thanks to everyone for pitching in!
OP, none of the answers here are satisfactory. Please ask the rangers and update this post because this is fascinating.
Google Lens wasn't helpful, it compares it to things like drains and fences built around old markers (it specifically thought it was a Mason-Dixon line marker which is obviously incorrect). But this is not protecting anything and not a drain.
My guess is, if it's not something to tie a horse to, is maybe it's something to put animal feed in? But that's a wild guess, I don't really think it's right. Like shove a hay bake in. But you said only accessible by walking, so I really doubt it.